Vinyl chloride resins, particularly those for use in pastes, are manufactured by a microsuspension polymerization process or an emulsion polymerization process.
In the microsuspension polymerization process, a monomer consisting of vinyl chloride or containing vinyl chloride as the major component is mixed with water, an emulsifying agent, a polymerization initiator soluble in the monomer, and other polymerization additives by means of high shear in an apparatus other than the polymerization apparatus to form a uniform dispersion. Subsequently the dispersion is transferred to the polymerization apparatus, and then the monomer is polymerized with stirring to yield vinyl chloride resin fine particles having an average particle diameter of about 0.2 to 3 .mu.m.
Although the dispersed monomer droplets formed in the mixing step are relatively stable because they are protected by the emulsifying agent, the droplets become unstable as the polymerization proceeds and reaches its latter half stage. If agitation during the polymerization is too strong, coalescence of particles resulting from collisions the particles is accelerated, so that the proportion of coarse particles becomes large and the amount of scale deposited on the wall of the polymerization apparatus or on the agitating element is increased, and in an extreme case, coagulation of polymer particles may occur and destroy the latex.
In addition, there are often cases where due to the increase in the amount of coarse particles, the particle diameter distribution is changed, and impairs flow properties, such as sol viscosity, of the product.
Therefore, in practicing the microsuspension polymerization process, low-shear type agitating elements are generally employed during polymerization. For this reason, the heat transfer coefficients of the jackets of the polymerization apparatuses have been low as compared with those for a suspension polymerization process employing vigorous agitation. Hence, the ability to remove heat of polymerization has been rate-determining for the attaining of improved polymerization production efficiency.
As the emulsion polymerization process, emulsion polymerization or seed emulsion polymerization are used. In emulsion polymerization, vinyl chloride monomer is polymerized in an aqueous medium with the aid of an anionic surface active agent and/or a nonionic surface active agent as an emulsifying agent and a water-soluble peroxide, a combination of a water-soluble peroxide and a water-soluble reducing agent, or a combination of an oil-soluble peroxide and a water-soluble reducing agent as a polymerization initiator and, if required, other polymerization additives, to yield vinyl chloride resin fine particles having an average particle diameter of 0.1 to 0.4 .mu.m. In seed emulsion polymerization, the above-described emulsion polymerization is conducted in the presence of, as seed particles, vinyl chloride resin particles that have been prepared beforehand, and as a result, the seed particles are enlarged to give relatively large particles having particle sizes of 0.4 to 2 .mu.m.
If the emulsifying agent is present in an excessive amount, very fine particles result. Hence, the emulsifying agent is additionally incorporated in the minimum amount necessary to cover the polymer particles precipitated, so that the particles are extremely unstable during polymerization. Therefore, relatively mild agitation is employed in the emulsion polymerization process as it is in the above-described microsuspension polymerization. It is, therefore, important also for the emulsion polymerization process to improve the ability to remove heat of polymerization in order to increase the production efficiency in the process.
When the microsuspension polymerization process or emulsion polymerization process as described above is carried out batch-wise, tank-type polymerization apparatuses having H/D ratios (where H is the effective height of the polymerization apparatus and D is the effective diameter thereof) of about 1 to 3 are generally used so that sufficient mixing in the upward and downward directions in the apparatuses can be obtained.
Various heat-removing methods have been proposed for such polymerization apparatuses to eliminate the problem concerning heat removal as described above. For example, there are a method in which heat of polymerization is removed by increasing the heat transfer area, a method in which the heat is removed by increasing the total heat transfer coefficient by, for example, changing the materials constituting the polymerization apparatus, or modifying the structure of the jacket and the shape of the agitating element, and a method in which the heat is removed by employing a low-temperature refrigeration medium to give an increased temperature difference.
As means of increasing the heat transfer area, it has been proposed, for example, to pass cooling water through the agitating element and baffles, to use a reflux condenser (JP-A-54-153894 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), to use an external cooling device (JP-A-55-157607), and to employ a polymerization apparatus equipped with a draft tube (JP-A-55-62908).
However, use of baffles is disadvantageous in that coalescence of particles is accelerated around the agitating element and baffles to increase the amount of coarse particles, and that the apparatus necessarily has a complicated structure, which results in an increase in the amount of scale deposits. Use of a reflux condenser in the microsuspension polymerization process is disadvantageous (although it is generally used in the emulsion polymerization process) in that scale deposition occurs due to the foaming of the liquid reactant mixture, and that the pressure increases abnormally and a bulk polymer is generated, due to insufficient introduction of the refluxed vinyl chloride monomer into the reactant mixture. Use of an external cooling device is disadvantageous in that latex is destroyed and the amount of coarse particles is increased due to the high shear in the circulating pump and scales are deposited in the cooling device. Thus, there are various problems to be solved for the practical use of this technique.
Polymerization is a polymerization apparatus equipped with a draft tube is conducted while the liquid reactant mixture is being circulated slowly by means of the draft tube under conditions that cause little turbulent flow, and this apparatus is effective in controlling scale formation. However, it is impossible to completely prevent scale deposition, and scales that are deposited are difficult to remove particularly from the space between the draft tube and the wall of the polymerization apparatus and from the lower part of the agitating element used for circulation. Thus, this polymerization apparatus is not suitable for long-term stable running.
Further, the method of removing heat of polymerization by increasing the total heat transfer coefficient is disadvantageous in that scales are apt to be generated, agitation conditions are limited due to the structure of the apparatus, and the apparatus is costly.
The method of removing polymerization heat by creating a large temperature difference is disadvantageous in that this method is costly because of the increased running cost of providing refrigeration.
With respect to the introduction of a monomer containing vinyl chloride into the polymerization apparatus is emulsion polymerization, the following methods, for example, have been proposed. In the examples of JP-A-57-98511, water and a water-soluble polymerization initiator are first introduced into the polymerization apparatus, the inside of the apparatus is then evacuated, vinyl chloride monomer is introduced, and then the temperature of the reaction mixture is raised to initiate polymerization, with an emulsifying agent being added continuously after the conversion to polymer reaches a certain level.
In JP-A-55-66504, water, a latex of seed polymer particles, and a reducing agent are first introduced into the polymerization apparatus, the inside of the apparatus is subsequently evacuated and vinyl chloride monomer is introduced, and then polymerization is initiated by raising the temperature of the reaction mixture and continuously adding a peroxide, with an emulsifying agent being added after the polymerization has proceeded to some extent.
As in the methods described above, it is common in the emulsion polymerization of vinyl chloride that all of the vinyl chloride monomer is introduced at one time at the beginning of the polymerization.
The emulsion polymerization is initiated when a free radical generated in the aqueous phase reacts with vinyl chloride monomer in the aqueous phase. Polymer particles precipitated are allowed to grow and maintain their predetermined particle diameters by additionally incorporating an emulsifying agent in an amount necessary to cover the polymer particles. The polymer particles (polymer latex particles) formed have a tendency to coagulate and destroy the latex and, if polymerization is conducted with agitation, the amount of scale which adheres to the wall of the polymerization apparatus and the amount of scale floating in the polymer latex are increased with increasing shear force resulting from the agitation. Therefore, the shear force produced by agitation should be restricted during polymerization in order to reduce the amount of such scale, and even if agitation is used during polymerization, it is essential to agitate at a low speed.
For example, the emulsion polymerization process described in JP-B-58-57409 (the term "JP-B" as used herein means an "examined Japanese patent publication") is characterized in that an initiator is introduced into the aqueous phase under such mild agitation conditions that the monomer phase and the aqueous phase are separated into two layers, in order to reduce the amount of scale formed during polymerization.
In the above method, however, mixing in the polymerization apparatus is insufficient because of the mild agitation. Since the rate of polymerization is determined by the rate of migration of the monomer to the aqueous phase, it is difficult to reduce the polymerization time if the agitation is not increased. If the amount of the polymerization initiator is increased in order to reduce the polymerization time, the resulting polymer has an exceedingly lowered average molecular weight. If the number of revolutions of the agitator is increased so as to promote diffusion of the vinyl chloride, there is a problem that the amount of coarse particles and the amount of scale deposits are increased.
Further, emulsion polymerization performed under low-rate agitation conditions causes the polymerization reaction system (the system in which both the monomer and the polymer latex are present) to have a high viscosity, which results in a low heat transfer coefficient at the interface between the polymerization apparatus and the polymerization reaction system and, hence, insufficient heat removal. A further problem is that the removal of heat of polymerization becomes even more difficult if a larger-size polymerization apparatus is employed or the proportion of introduced vinyl chloride monomer introduced into the apparatus is increased.
In order to solve the heat removal problem, it has been proposed to pass a low-temperature brine through the jacket of the polymerization apparatus, or to provide a cooling coil in the polymerization apparatus. However, the former proposal is disadvantageous in that the production cost is increased because of increased power consumption, while the latter proposal is disadvantageous in that deposition of polymer particles occurs on the cooling coil, and this necessitates cleaning to remove the deposited polymer. Thus, neither of these proposals is preferable.
On the other hand, removing heat of polymerization by means of a condenser provided in the gas-phase section of the polymerization apparatus is known in the suspension polymerization of vinyl chloride and in other polymerization processes. This heat-removing technique can generally be employed only where the polymerization reaction system is agitated at a high speed and is in a completely mixed state. According to this technique, vinyl chloride monomer is condensed in the condenser and continuously returned to the part above a surface of the reactant mixture in the polymerization apparatus through a nozzle, as described, for example, in JP-B-58-48561.
However, in the production of vinyl chloride resins for use in pastes, it is impossible to employ high-speed agitation. If, for this reason, emulsion polymerization to produce a vinyl chloride resin is performed under low-speed agitation conditions while the monomer is being condensed and returned to the gas phase by means of a condenser, the result is that the polymerization reaction system is poorly mixed. Also, because of the latent heat of vaporization of the monomer which is present in a large amount in the upper part of the polymerization apparatus, only the upper part of the liquid phase is cooled and a temperature difference arises between the upper and lower parts of the liquid phase. As a result the heat-removing efficiency of the condenser is lowered.